The present invention relates to an image display apparatus and, more particularly, to an easy-to-view head-mounted image display apparatus in which images are displayed under conditions close to those of the human visual field, thereby enabling the displayed images to be favorably seen without impairing the dynamic presence even if the image field is narrower than the field of view.
In recent years, head-mounted display apparatuses (HMD: Head Mounted Display) have been proposed in the field of virtual reality, in which the virtual image of a small-sized display disposed in front of an observer's eye is projected onto the retina of the eyeball by an optical system also disposed in front of the observer's eye, thereby allowing the observer to perceive the image. This type of apparatus is expected to be used in various applications. For example, a user wears such a display apparatus of his/her head to enjoy a wide-screen picture or image and sound of VTR personally.
Incidentally, when the image of an image display device is led to an observer's eye by an ocular optical system in the head-mounted image display apparatus, if the periphery of the projected image field is clearly displayed, the boundary between the image display area and the non-image display area, that is, the frame of the image field, is recognized. Accordingly, the displayed image field shows up clearly against a dark background. The frame separates distinctly the world of the displayed image from the observer's real world, causing the dynamic presence of the displayed image to be impaired to a considerably extent. If the frame of the image field, that is, the boundary, is clearly displayed with a strong contrast, the observer's line of sight tends to be directed toward the outer peripheral portion of the image field by the psychological effect, causing the observer to be unwillingly conscious of the presence of the frame, and thus making it difficult to obtain the dynamic presence of the displayed image.
The display condition that a bright image field shows up clearly against a dark background is unfavorable from the viewpoint of the fatigue of the observer's eye.
The frame of the image field, which causes degradation of the dynamic presence and aggravation of the fatigue, consists mainly of the images of the edges of the image display device and the image of the field stop.
To make the image field sufficiently large to prevent the frame from entering the field of view or to prevent the frame from interfering with the observation of the displayed image even if it is within the field of view, a wide observation field angle of from 120 degrees to 160 degrees or more is needed. To meet the demand of such a wide field angle, the optical system must become complicated and larger in size to correct aberrations. Moreover, a two-dimensional image display device corresponding to the wide field angle is required. However, a restriction in size is imposed on an image display apparatus of the type that is mounted on the user's head. Therefore, it is difficult to realize such a wide field angle under the restriction on the size of the apparatus.
To resolve these problems, Japanese Patent Application Unexamined Publication Number [hereinafter referred to as "JP(A)"] 8-146339 discloses a technique wherein a peripheral light source capable of independently illuminating the periphery of the displayed image is provided separately from illumination for an image display unit, and a mechanism for adjusting the peripheral light source is also provided, thereby giving a luminance to the periphery of the displayed image. JP(A) 5-328258 discloses a technique wherein a milky white diffuse transmission curved plate or the like is provided in an optical system to make the periphery of the displayed image unsharp. JP(A) 7-325266 discloses a technique wherein a light-blocking member or the like is provided in an optical system at a position corresponding to the periphery of the displayed image so that the periphery of the image is gradually blurred to become darker and unsharper.
According to these conventional techniques, light from an image display device, which is provided from the beginning, is not led to the periphery of the displayed image, but light from a newly provided light source is led to the periphery of the image, or marginal light is intentionally vingetted, thereby making the periphery of the image unsharp. Therefore, light that carries image information is not led to the unsharp portion outside the image display area. Accordingly, even when the image itself changes to a bright scene, the periphery of the image may be still dark. Conversely, the periphery may be bright despite a dark scene. Thus, the peripheral portion does not change in association with the displayed image and hence involves some unnaturalness. Further, the conventional techniques are not intended to widen the field angle effectively.
Regarding optical systems for use in head-mounted image display apparatuses, there have been proposed an optical system using a half-mirror and a concave mirror, and a prism-type optical system (JP(A) 6-268944). These optical systems are arranged to obtain a wide field angle for the size thereof while meeting the demand to achieve a reduction in the size of head-mounted image display apparatus, which is one of the requirements for this type of image display apparatus.
Regarding the vertical direction of these optical systems, as shown in FIG. 64, light from an image display device 1 passes through a half-mirror 3 to impinge on a concave mirror 2, and the reflected light is reflected by the half-mirror 3 and projected into an observer's eyeball E. The concave mirror 2 and the half-mirror 3 are disposed such that their center axes intersect each other at approximately 45 degrees. Therefore, the optical system has an approximately cubic configuration in which the effective diameter a of the concave mirror 2 is approximately equal to the length b of an exit surface 4, which is a surface closest to the observer's eyeball E. The exit surface 4 is not present if the optical system consists of only the concave mirror 2 and the half-mirror 3. However, a space shown by S in FIG. 64 is physically needed on the eyeball side. When the optical system is actually used as a product, the exit surface 4 is the surface of a cover glass. In the case of a prism optical system, the exit surface 4 is an exit-side surface of the prism. FIG. 65 shows above-described arrangement as an equivalent optical system which does not use the half-mirror 3. A refracting lens 5 in FIG. 65 has a positive refracting power equivalent to the power of the concave mirror 2 and is therefore equivalent to the concave mirror 2. In this case, the concave mirror 2 is a principal refracting surface having an action by which a bundle of light rays is bent to lead light rays to the observer's eyeball E. In the equivalent optical system, the concave mirror 2 is obviously the lens 5. As will be understood from FIGS. 64 and 65, because the refracting surface and the exit surface 4 are at a distance from each other, light beams passing via the edge of the concave mirror 2, or the lens 5, which limits the numerical aperture, do not pass through the edge of the exit surface 4, which is approximately equal in size to the refracting surface owing to the arrangement of the optical system, but pass through a portion closer to the optical axis than the edge. The same is true of the horizontal direction (direction perpendicular to the plane of the figure) of the optical system. That is, at the exit surface 4, the space S lying between the portion through which the light beams pass and the edge of the exit surface 4 is a dead space that is not used.
Thus, the conventional optical systems comprising a combination of the half-mirror 3 and the concave mirror 2 have the advantage that a wide field angle for observation is realized despite a compact structure, but in some respects they have the dead space S at the exit surface 4.